High-frequency dynamics of vitreous GeSe2

The vibrational dynamics of vitreous germanium diselenide (v-GeSe2), an amorphous glass belonging to the family of continuous random network forming glasses, has been investigated by means of inelastic neutron scattering. We employed three different neutron spectrometers, each of them providing different energy resolutions and spanning different portions of the energy-wave-vector ((h) over bar omega,Q) plane, which partially overlap so that complementary information can be obtained. Two well-defined peaks are resolved in the dynamic structure factor in a broad Q region, below and above the position of the first sharp diffraction peak in the static structure factor. The low-lying excitation is nondispersive and located around the boson peak frequency. The high-frequency mode presents a pseudoperiodic behavior and it can be associated to the high-frequency counterpart of the longitudinal acoustic (LA) mode. In the low-Q region this mode shows a positive dispersion of its apparent sound velocity and an attenuation which follows a Q(2) law. The combined use of a set of appropriate instruments allows an accurate determination of the nonergodicity parameter f(Q). The LA dispersion curve of v-GeSe2 is similar to that measured in other glasses where a second excitation appears in the spectra. On the contrary other glass forming materials, where this second mode is not detected, present a sinusoidal-like dispersion curve with a deeper minimum in the second pseudo-Brillouin zone.